Out of place hydration using render and eval

[phanimarupaka]

Commit 1:

1. Add --output(-o) flag to render and eval commands, to support out of place hydration and writes rendered resources to stdout(stdout wrapped)|unwrap(stdout unwrapped)|OUT_DIR_PATH(to another directory). (kpt fn render should support out-of-place sink-mode #1412)
2. Deprecate --dry-run flag from eval command as it is supported by --output flag.

Commit 2
Add e2e tests for oop hydration

Commit 3
Reference docs.

Supported example

kpt fn render [HERE] -o stdout \
| kpt fn eval --image gcr.io/kpt-fn/set-annotations:v0.1.3 -- foo=bar \
| kpt fn eval --image gcr.io/kpt-fn/set-labels:v0.1.3 -o [stdout|unwrap|OUT_DIR]-- tier=backend

Note: render can only read from directory and not STDIN. eval can read from both stdin and directory.

[frankfarzan]

This PR is doing too many things, and combines multiple UX changes each deserving their own consideration e.g (Using dry-run in render for output) and e2e testing. Let's split this up into multiple orthogonal PRs.

[morgante]

@phanimarupaka Thanks for working on this! 💯 From a cursory look it seems this would indeed unblock the hydration functionality needed for LZ.

[frankfarzan]

Please add the different usages to the reference docs and make sure they have e2e tests. To make this manageable consider splitting into separate PRs, one per command.

[droot]

Did a quick pass of the Go code only.

[droot]

Second pass only on the Go code, will review the docs/tests in the morning.

[droot]

Looking in good shape, few more comments.

[frankfarzan]

/lgtm with the suggested follow up on non-structured results. Please wait for @droot to approve.

[droot]

Thanks for working on this. This is good to go 🚢

We need to follow up with:

1. We need to update the go/kpt-testcases sheet for out-of-place
2. non-structructured output to be send to stderr always: that will remove conditional printing
3. Removing Stdin (-) from fn eval

[frankfarzan]

4. Source using -output consistently.
5. Outputting meta resources (cross-cutting).